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For further information, see CMDT Part 20-21: Microscopic Polyangiitis

Key Features

Essentials of Diagnosis

  • Necrotizing vasculitis of small- and medium-sized arteries and veins

  • Most common cause of pulmonary-renal syndrome: diffuse alveolar hemorrhage and glomerulonephritis

  • Associated with antineutrophil cytoplasmic antibody (ANCA) in 75% of cases

General Considerations

  • A pauci-immune nongranulomatous necrotizing vasculitis that

    • Affects small blood vessels (capillaries, venules, or arterioles)

    • Often causes glomerulonephritis and pulmonary capillaritis

    • Often is associated with ANCA on immunofluorescence testing (directed against MPO, a constituent of neutrophil granules)

  • Appears in a spectrum overlapping with both polyarteritis nodosa and granulomatosis with polyangiitis (formerly Wegener granulomatosis) because it

    • May involve medium-sized as well as small blood vessels

    • Tends to affect capillaries within the lungs and kidneys

  • Rarely, medications induce a systemic vasculitis associated with high titers of p-ANCA and features of microscopic polyangiitis, particularly

    • Propylthiouracil

    • Hydralazine

    • Allopurinol

    • Penicillamine

    • Minocycline

    • Sulfasalazine

Clinical Findings

Symptoms and Signs

  • "Palpable" (or "raised") purpura and other signs of cutaneous vasculitis (ulcers, splinter hemorrhages, vesiculobullous lesions)

  • Pulmonary-renal syndromes (eg, anti-glomerular basement membrane disease)

  • Interstitial lung fibrosis that mimics usual interstitial pneumonitis

  • Pulmonary hemorrhage may occur

  • Vasculitic neuropathy (mononeuritis multiplex)

Differential Diagnosis

  • Polyarteritis nodosa

  • Granulomatosis with polyangiitis


Laboratory Tests

  • Microscopic hematuria, proteinuria, and red blood cell casts may be present in the urine

  • Elevated acute-phase reactants are typical of active disease

  • 75% of patients are ANCA-positive, usually with anti-myeloperoxidase antibodies (MPO-ANCA) that cause a p-ANCA pattern on immunofluorescence testing

  • ANCA directed against proteinase-3 (PR3-ANCA) can also be observed

  • Pathologic findings in the lung are typically those of capillaritis

  • The renal lesion is a segmental, necrotizing glomerulonephritis, often with localized intravascular coagulation and the observation of intraglomerular thrombi upon renal biopsy



  • Treated in the same way as granulomatosis with polyangiitis

  • Patients with severe disease, typically involving pulmonary hemorrhage and glomerulonephritis, require urgent induction treatment with corticosteroids and either cyclophosphamide or rituximab

  • If cyclophosphamide is chosen, it may be administered either in an oral daily regimen or intravenously

  • Following successful induction of remission, maintenance treatment should be continued with rituximab (first line) or methotrexate or mycophenolate (second line)

  • In cases of drug-induced MPO-ANCA-associated vasculitis,

    • The offending medication should be discontinued

    • Significant organ involvement (eg, pulmonary hemorrhage, glomerulonephritis) requires immunosuppressive therapy



  • Early diagnosis is key to a good outcome

  • Compared with patients who have granulomatosis with polyangiitis, those with microscopic polyangiitis are more likely to have significant fibrosis on renal biopsy because of later diagnosis

When to Refer

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